Electric detonating apparatus



J 2, 1968 H. JOHNSTON ET AL 3,361,064

3 ELECTRIC DETONATING APPARATUS Filed Sept. 7, 1950 2 Sheets-Sheet 1 I WITNESSES: .INVENTORS Lawrence H. Jolznsfion Jan. 2, 1968 H. JOHNSTON ETAL 3,

ELECTRIC DETONATING APPARATUS Filed Sept. 7, 1950 2 Sheets-Sheet 2 Q N I o 73 .g I

I 1 25? 0 30 I m0 O l w a '3 I i ,3 A L w? J 8 a 5* f/ u i Q M 8 o O N to N O o O 0 O F "0 N N 06 WITNESSES INVENTORS Lawrence H. Joknsi'ozz W Z RBci bez'z Allolreclge United States Patent 3,361,064 ELECTRIC DETONATING APPARATUS Lawrence H. Johnston, Minneapolis, Minn., and Robert Alldred'ge, Denver, Colo., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Sept. 7, 1950, Ser. No. 183,586 5 Claims. (Cl. 10228) This invention relates to detonators for explosives and more particularly to detonators of the spark gap type. It also relates to a method of manufacturing detonators.

Most useful explosives are materials and compositions within which coexist a fuel and a source of oxygen for the burning of the fuel, so that, when initiated, the resulting fire is self-sustaining and proceeds in a manner substantially independent of the surroundings. Two classes of explosives are recognized: those in which the fire occurs as a progressive burning not unlike the familiar burning of readily combustible materials, and those in which the fire passes through the mass in the form of a wave traveling at a rate of the order of thousands of feet per second. The rate of energy release is obviously very much greater for the second class whose members are termed high explosives as opposed to the low explosives of the first class. Low explosives are employed in the pro pelling of projectiles; high explosives are used for blasting, demolition, filling military projectiles, and the like.

High explosives are again divided into two classes, sensitive and insensitive. In order to achieve certain desired results the employment of large masses of high explosive is often necessary. If detonation of these large masses can be readily effected, safety problems are severe. Consequently, insensitive high explosives must be available for use as main charges, and sensitive explosives are needed in small amounts for initiating the detonation of these main charges. High explosives sensitive to small amounts of energy are termed primary explosives.

Certain among the primary explosives are exceptions to the general oxygen-fueldefinition of explosives: these primary explosives are unstable chemicals, such as lead azide, mercury fulminate, copper acetylide, and diazo compounds, in which the readily released energy is present in the form of energy of intramolecular configuration.

For safety and convenience, a high explosive charge usually consists of two or three components: a primary explosive in very small amount, a man charge in large amount, and, if desired, an intermediate or booster charge, consisting of high explosive of intermediate sensitivity, which serves to amplify and transmit the readilyinitiated detonation wave of the primary explosive so as to cause the detonation of the insensitive main charge. There are many convenient methods for supplying small amounts of energy to initiate detonation of the primary explosives; the most convenient means for supplying the large amounts of energy required for setting off the insensitive main charges is the detonator, comprising primary explosive, booster charge and energy supply for setting off the primary explosive. Detonators are thus seen to be highly important to the explosives art.

There are certain disadvantages common to detonators of the prior art. Chief among these is the large and nonuniform time lag which occurs between the supplying of energy to the detonator and the detonation of the primary explosive therein. This irregular time lag is a highly important factor in many operations where simultaneity of explosion among a number of detonators is required. It results largely from the varying densities and dimensions of the primary explosive in different detonators. Prior to this invention however, no method of manufacturing detonators by which the dimensions and density of the primary charge could be accurately controlled has been available.

Another disadvantage attendant on the use of known detonators is the fact that the primary explosive as used therein in the loosely packed form is quite sensitive to detonation by impact.

A disadvantage peculiar to the bridge wire detonators employing a primary explosive is that detonation can be caused by accidentally or inadvertently applied charges of electricity of ordinary low voltage. For example, the potential present in storage batteries or ordinary utility circuits is often adequate to heat the bridge wire sutficiently to ignite the surrounding explosive even though the bridge wire itself is not melted or exploded.

It is therefore an object of this invention to provide a detonator which is insensitive to electrical impulses of ordinary low voltages.

It is another object of this invention to provide a detonator in which the time lag is uniform and is reduced to a minimum.

It is still another object of this invention to provide a detonator which can be detonated by a small amount of electrical energy.

It is a further object of this invention to provide a detonator which is highly insensitive to explosion by impact.

It is a still further object of this invention to provide a method of manufacturing detonators by which the density and dimensions of the primary charge can be accurately controlled.

It has been found that the above objects relating to the product can be accomplished according to this invention by the combination comprising an outer case, spark gap electrodes maintained in spaced relationship within the case to form a spark gap, a primary explosive compacted in the spark gap, means associated with the detonator for draining off charges of static electricity and means for sparking the gap. The object relating to the method is accomplished by a process for manufacturing the detonator which comprises first securing electrodes in the detonator case at one end, inserting explosive from the open end and compressing the explosive in the case, under the desired pressure.

Tests have shown that as a primary explosive in the spark gap becomes more highly compressed the firing delay of the detonators for a given energy threshold is lowered. It is believed that this is not due to the explosive becoming more sensitive to electrical energy as it is compressed, since most explosives become less sensitive under these conditions. Rather, it appears that the spark mechanism in the detonator becomes a more emcient means of dissipating electrical energy when the gap is filled with a compressed primary explosive such as lead azide.

The invention can best be understood by reference to the accompanying drawings which are hereby made a part of this specification.

FIGURE 1 is an illustration of the apparatus of the invention with the detonator shown in vertical cross section.

FIGURE 2 is a family of curves showing firing delay versus firing energy for various pressures used to compress lead azide in the spark gap of the detonator of the invention.

Referring particularly to FIGURE 1, the detonating apparatus comprises a container or shell 10, of metal, plastic or like material. At 11 is shown a supporting plug of plastic or other dielectric material which seals the lower end of container 10. The plug 11 support in spaced relationship the electrodes 12 and 13 whose upper ends extend through the upper end of the plug to form a spark gap and whose lower ends extend through the lower end of the plug for electrical connection. A charge of primary explosive 14 is pressed into the space directly above the plug 11 and in the spark gap formed by the upper ends of electrodes 12 and 13. A base charge 15 of high explosive such as PETN (pentaerythritol tetranitrate) or RDX (cyclotrimethylene trinitramine) is located adjacent the primary charge or if necessary a booster charge of a material such as tetryl (trinitrophenylmethylnitramine) may be placed between the primary and base charge. An electrical circuit suitable for sparking the gap comprises a condenser 16 connected to lead-in wires 17 and 18 through a switch 19, which in one position connects the condenser to a source of high voltage such as battery 21 and in the other position discharges the condenser 16 across the spark gap formed by electrodes 12 and 13. For preventing the accumulation of charges of high voltage due to static electricity resistance 22 is placed in parallel with the spark gap.

Lead azide which has a decomposition temperature of 180 C. is preferably used as the primary explosive as its thermal stability is quite good. Other primary explosives may obviously be used, for example, it has been found that dextrinated lead azide is quite suitable. Primary explosive as used in this specification has its ordinary meaning, namely, a sensitive type of high explosive as distinguished from insensitive high explosive such as PETN, RDX and TNT (trinitrotoluene).

A controlling feature of the invention is the degree to which the explosive is compressed in loading. When lead azide is used as the primary explosive, almost all properties of the detonator improve as the pressure of loading is increased up to a certain point. For instance, at low pressure (about 100 lb. per square inch) the energy threshold is quite sensitive to the geometry of the gap and there is some evidence that it is also sensitive to the method of preparation of the explosive. At higher pressures, however, in the range 5,00025,000 lb. per square inch, almost any gap geometry may be used and all tests of lead azide pressed within this range gave uniformly good timing and sensitivity.

The effect of pressure on firing delay is shown by the curves of FIGURE 2 made from results of a number of tests. These curves show that for a given firing delay the energy required for successful firing of the device decreases markedly as the pressure used to compress the azide increases. This holds true up to about 25,000 lb. per square inch. Above 25,000 lb. per square inch there is a general leveling off of the decrease in firing delay. The measurements on which the graph is based were made by the standard DAutriche method.

The beneficial effects of high pressures for compressing the primary explosive are not limited to a decrease in firing delay. Increased pressures effect a marked decrease in the impact sensitivity of the detonator as shown by results obtained from drop tests using an 80 lb. steel cylinder falling from various heights. A large number of consistent results showed that, in the case of lead azide, sensitivity decreases regularly up to 15,000 lb. per square inch but that between 15,000 and 25,000 lb. per square inch the decrease in sensitivity to impact is proportionately greater than in any other range and in the neighborhood of 20,000 lb. per square inch this sensitivity is less than at either 15,000 or 25,000. Above 25,000 lb. per square inch the sensitivity is not markedly affected by increased pressure. It appears that the range of pressures between 15,000 and 25,000 lb. per square inch and in the neighborhood of 20,000 is critical for producing the highest resistance to explosion by impact.

The use of a uniformly compacted explosive in the spark gap results in a uniform time lag so that reproducibility of timing can be achieved, thus permitting simultaneity of explosion among a number of detonators. In addition, the use of compressed high explosive the spark gap reduces the time lag itself to a minimum.

To further insure uniformity of timing among a number of detonators it is important that substantially identical amounts of explosive be compressed in each detonator not only under substantially equal pressures but to substantially identical dimensions. This procedure insures that the distance A as shown in FIGURE 1 between the plane in which the spark travels across the gap and the upper face of the primary explosive is substantially the same for all detonators of a group. Detonators of the prior art are highly deficient in this respect. This lack is probably due in part to the fact that prior to this time there has been very little demand for a detonator having a delay in microseconds or for simultaneity within fractions of microseconds among a number of detonators and consequently previous methods of manufacture are not capable of providing such a detonator. This is especially true with respect to the method of loading prior art detonators which is necessitated by the construction of the detonators.

Detonators of the prior art such as those disclosed in Patent No. 2,360,698 to Lyte employing electrodes made integral with a removable plug require that the electrodes be placed in position after the explosives have been inserted. This makes it impossible to apply pressure to the explosive surrounding the spark gap so that, as a result, prior art detonators have not had a uniform time lag or a time lag in microseconds. Conversely, in manufacturing the detonator of this invention the electrodes are secured in one end of the outer case first and the explosives loaded from the open end. By using this novel pr0- cedure accurately controlled pressures can be applied to the explosives during loading so that accurately controlled dimensions can be achieved. This process provides detonators containing substantially identical amounts of explosive compressed under equal pressures to substantially identical dimensions, thereby contributing materially to the attainment of uniformity in time lag among detonators.

The container or shell of the detonator may be made of copper or other suitable material. The geometry of the gap and container is preferably such that the spark is forced to travel through explosive rather than along an interface between the explosive and the container. This construction has been found to improve the efliciency of the spark gap. Although a spark gap arrangement of two electrodes as disclosed is the preferred embodiment, other arrangements may be used. For example, a spark gap in which the detonator case serves as one electrode may be used or more than two electrodes may be used.

The electrodes may be made of copper, aluminum or other suitable material. For use with lead azide, aluminum is preferred to avoid the formation of sensitive compounds such as copper azide which is often formed when copper is in contact with lead azide. The electrodes may be of almost any shape which limits the spark to a definite plane in the explosive. The spark gap spacing is desirably kept small in order to lower the minimum sparking potential and to limit the spark to a narrow plane. A spacing of between 0.015 and 0.025 inch was found to be the most desirable from a practical standpoint, this spacing requiring a potential of from 2000 to 4000 volts. In the ininterest of safety the characteristics of the gap with the pressed explosive therein should be such that at least 1000 volts are required to spark the gap, thus making the detonator safe against the inadvertent or accidental application of ordinary low voltages as, for example, the potential present in storage batteries or ordinary utility circuits.

A mechanical spark switch is generally used in the circuit; however, a triggered spark gap or similar arrangement is equally effective. For simultaneity shots good timing was achieved with as little capacitance as 0.002 mfd. per detonator. The circuits used had an inductance of about 3 microhenries per detonator; however, much higher inductances may be used without reducing the accuracy of timing. In general, it is desirable to use a voltage at least 50 percent higher than the minimum sparking potential of the gap. The most satisfactory voltage, from a practical standpoint, was found to be in the neighborhood of 5600 volts. In order to guard against detonation from high voltages accumulated from static electricity a resistance is maintained in parallel with the spark gap at all times. This will drain off charges accumulating from static electricity but will not efiect the firing pulses. A 500 ohm resistor has been found satisfactory.

Detonators made in accordance with this invention are found to be safe against the application of ordinary low voltages and to be highly resistant to detonation by impact. Their reproducibility of timing is excellent and in simultaneity tests a spread of from 0.08 to 0.2 microsecond in timing is consistently obtained.

As many apparently widely difierent embodiments of this invention may be made Without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.

What is claimed is:

1. Apparatus for detonating high explosive in uniform timing comprising in combination, an outer case, spark gap electrodes insulatedly supported in spaced relationship therein to form a spark gap, at primary explosive of the class consisting of lead azide and dextrinated lead azide compressed under a pressure in the neighborhood of 20,000 lbs. per square inch in said spark gap, means connected in parallel with said gap for draining ofi charges of static electricity and means for sparking sai-d spark gap.

2. Apparatus for detonating high explosive in uniform timing comprising in combination, an outer case, spark gap electrodes insulatedly supported in spaced relationship therein to form a spark gap, a primary explosive of the class consisting of lead azide and dextrinated lead azide compressed under a pressure in the neighborhood of 20,000 lbs. per square inch compacted in said spark gap, said spark gap with said compressed explosive therein requiring at least 1000 volts for sparking, means associated with said detonator for draining oif charges of static electricity and means for impressing at least 1000 volts on said spark gap.

3. The apparatus of claim 1 in which the explosive is lead azide.

4. The apparatus of claim 1 in which the explosive is dextrinated lead azide.

5. The apparatus of claim 1 in which the explosive is lead azide and in which the electrodes are spaced apart a distance of between 0.015 and 0.025 of an inch requiring a potential of from 2000 to 4000 volts for sparking.

References Cited UNITED STATES PATENTS 97,241 11/1869 Smith l02-28 347,013 8/1886 Scola 102-28 2,404,553 7/ 1946 Wales 102--70.2

FOREIGN PATENTS 868,529 10/1941 France.

OTHER REFERENCES Davis, Chemistry of Powder and Explosives, vol. II, pp. 410 and 424, John Wiley & Sons, New York (1943).

Ohart, Elements of Ammunition, p. 34, John Wiley & Sons, New York (1946).

BENJAMIN A. BORCHELT, Primary Examiner.

JAMES L. BREWRINK, ROGER L. CAMPBELL,

Examiners.

E. CHESLOW, A. P. KENT, V. R. PENDEGRASS,

Assistant Examiners. 

1. APPARATUS FOR DETONATING HIGH EXPLOSIVE IN UNIFORM TIMING COMPRISING IN COMBINATION, AN OUTER CASE, SPARK GAP ELECTRODES INSULATEDLY SUPPORTED IN SPEED RELATIONSHIP THEREIN TO FORM A SPARK GAP, A PRIMARY EXPLOSIVE OF THE CLASS CONSISTING OF LEAD AZIDE AND DEXTRINATED LEAD AZIDE COMPRESSED UNDER A PRESSURE IN THE NEIGHBORHOOD 